What is the name of this type of photos where all the characters are shown?
If you search for movie posters with several characters you'll find it's usually called a Montage or Character Montage. There's even a super hero meme montage generator so apparently it's also a meme somehow.
It seems that the type of composition doesn't matter, I've seen different arrangements and it's still called a montage. This type of composition used to be called simply a collage up until I googled it.
Addendum: according to the Merriam-webster dictionary, a Montage is (emphasis mine)
- : the production of a rapid succession of images in a motion picture to illustrate an association of ideas
- a : a literary, musical, or artistic composite of juxtaposed more or less heterogeneous elements
b : a composite picture made by combining several separate pictures